This invention relates to button-size electrochemical cells, and especially to metal-air cells of the zinc-air type. Such cells are used for a variety of applications. A large fraction of such cells are used in hearing aids. Newer versions of such hearing aids are placed inside the outer portion of the human ear, whereby any leakage of material from the cell may come into contact with the skin of the wearer, in the wearer's ear. Accordingly, it is critical that such cells not leak any of the alkaline electrolyte which is routinely used in such cells.
Further, to the extent such leakage is detected in the manufacturing process, quality control efforts must be expended to reduce or preferably eliminate such leakage wherever practical. Such quality control efforts, of course, bear associated costs. Therefore, any advance in reducing leakage of electrolyte is significant to the health of the consumer of such cells, as well as to the commercial value of such cells to the manufacturer.
Further, the overall volume of the void space inside the cell, useful for containing electrochemically reactive materials, is limited to that space not occupied by non-reactive components of the cell. Thus, to the extent use of internal void space by non-reactive components can be controlled or reduced, additional electrochemically reactive material may be used in the cell, and the use life of the cell thereby extended. Accordingly, any effort expended in controlling leakage must be accompanied by a sensitivity to any reduction in the space which is available for use in containing electrochemically reactive materials in the anode can.
One potential source of leakage is leakage past the seal which separates side walls of the anode can and the cathode can. Such seal must participate in the equivalent of a frictional engagement with both side walls and associated sealing fluids, or other materials, in order to successfully provide the necessary seal function. Such frictional engagement with the anode can is routinely obtained by a sliding assembly of the anode can and the seal to each other, wherein, in conventional such assemblies, the (plastic) seal may be damaged by the anode can. Such damage is difficult to detect, and thus such damaged cells may well leave the manufacturing facility undetected.
Further, during final assembly of the cell, pressure exerted by the distal edge of the anode can against the seal, and indirectly against the underlying cathode assembly, causes that portion of the cathode assembly which is disposed inwardly of the seal to rise toward the top wall of the anode can (doming), and thus to somewhat withdraw from its location adjacent the bottom wall of the cathode can. Such doming is desirable to the extent it facilitates formation of an adequately-sized air reservoir between the cathode assembly and the bottom wall of the cathode can. However, the amount of doming corresponds with a respectively reduced amount of the anode volume being available for receiving and containing electrochemically reactive anode material in the anode can. Therefore, it is highly desirable to configure the respective elements of the cell, in cooperation with the assembly process, so as to closely control doming of the cathode assembly toward the top wall of the anode can to accommodate efficient formation of the air reservoir while minimizing the amount of the anode volume which is thus occupied by such movement of the cathode assembly during the cell assembly process.
It is an object of this invention to provide improved anode cans, and electrochemical cells made therefrom, wherein the cells have less tendency to leak electrolyte, and wherein a high fraction of the anode void volume of a conventional anode can is retained while improving control of doming of the cathode assembly.
It is another object to provide an anode can having a normal full-size perimeter over the major portion of the side wall of the anode can, and a reduced perimeter edge region adjacent the distal edge thereof.
Yet another object is to provide an anode can wherein the outer surface of the edge region of the anode can side wall is displaced inwardly of the outer surface of the anode can side wall as defined at locations away from the edge region.
A further object is to provide an anode can wherein the edge region is displaced inwardly of an intermediate region of the side wall of the anode can, and wherein the average thickness of the side wall between the intermediate region and a distal edge of the side wall is less than twice the thickness of the side wall in the intermediate region.
It is still another object to provide electrochemical cells made with the above anode cans.